The present invention relates to drilling systems and operations. More particularly, the present invention is a method and system for handling formation fluids entering the wellbore in the form of a hydrocarbon gas during well events.
Drilling fluids, also known as muds, cool the drill bit, flush the cuttings away from the bit's formation interface and then out of the system, and stabilize the borehole with a "filter cake" until newly drilled sections are cased. The drilling fluid also performs a crucial well control function and is monitored and adjusted to maintain a pressure with a hydrostatic head in uncased sections of the borehole that prevents the undesired flow of pressured well fluids into the borehole from the formation during drilling operations, i.e., well control events.
Conventional offshore drilling circulates drilling fluids down the drill string and returns the drilling fluids with entrained cuttings through an annulus between the drill string and the casing below the mudline. A riser surrounds the drill string starting from the wellhead at the ocean floor to drilling facilities at the surface and the return circuit for drilling mud continues from the mudline to the surface through the riser/drill string annulus.
In this conventional system, the relative weight of the drilling fluid over that of seawater and the length of the riser in deepwater applications combine to exert an excess hydrostatic pressure in the riser/drill string annulus.
Systems have been conceived to bring the drilling fluid and entrained cuttings out of the annulus at the base of the riser and to deploy a subsea pump to facilitate the return flow through a separate line. One such system is disclosed in U.S. Pat. No. 4,813,495 issued Mar. 21, 1989 to Leach. That system requires complex provisions to ensure the closely synchronous operation of the supply and return pumps critical to the approach disclosed. However, the durability and dependability of such a mud circulation system is suspect in the offshore environment and particularly so in light of the incompatibility of the fluid with pumping operations following a well control event.
Thus, there remains a need for technology facilitating subsea pump operation for the return of drilling fluid to the surface.